Comparison of stator winding connections in multiphase drives under healthy operation and with one open converter leg

Multiphase machines with symmetrical windings offer various stator winding configurations (SWCs). It is known that, for a given five-phase machine, the pentagon SWC implies, compared with star, smaller voltage rating and greater current rating of the converter. For healthy conditions, the stator copper loss (SCL) and maximum achievable torque (MAT) are almost identical for both SWCs, provided such converter-rating adjustments and negligible circulating currents. Regarding the operation of five-phase machines under open converter leg (especially common fault), the pentagon SWC attains lower SCL (for given torque) and larger MAT than star. However, it is unknown if these characteristics hold (and to which extent) for other phase numbers and SWCs. This study compares for multiple phase numbers the SWCs in terms of converter rating, MAT and SCL, under healthy operation and with an open converter leg. Most importantly, it is shown that, for phase numbers higher than five (the case previously studied), the improvement in performance (especially postfault MAT) by resorting to SWCs other than star can be much larger than for five phases, and it is thus of considerable interest. The most convenient alternatives (often different from the five-phase case) are established. Experimental results with two multiphase drives are included

DC-current injection with minimum torque ripple in interior permanent-magnet synchronous motors

Several proposals based on dc-current injection have been reported for estimating the stator winding resistance in induction machines, and recently extended for synchronous machines. Tracking this resistance can be very useful, e.g., for thermal monitoring or preserving control dynamics. In surface-mounted permanent-magnet synchronous machines (PMSMs), it is possible to inject a dc component in the d -axis, without perturbing the torque. However, it has been claimed that, for synchronous machines with saliency, it is not possible to avoid the torque ripple due to such injection. This letter proposes optimum reference currents to impose dc current in three-phase interior PMSMs while minimizing to practically zero its associated torque ripple. Namely, the dc signal is injected in combination with a suitable second-order harmonic so that the stator current space vector follows the constant-torque locus, while the fundamental is set according to the maximum-torque-per-ampere strategy. Experimental results validate the theory.Agencia Estatal de Investigación | Ref. DPI2016-75832-

Enhanced resonant current controller for grid-connected converters with LCL filter

Conventional resonant controllers (RCs) are commonly used in the current control of grid-tied converters with LCL filter due to their advantages, such as zero steady-state error at both fundamental sequences, easy design process, and straightforward implementation. Nevertheless, these traditional solutions do not permit to place the closed-loop poles of the system in convenient locations when dealing with a fourth-order plant model such as the LCL filter plus the computation delay. Therefore, the reference tracking and the disturbance rejection are deficient in terms of transient behavior and depend on the LCL filter. Furthermore, an additional active damping method usually has to be designed in order to ensure stability. This paper presents an enhanced current RC with stable and fast response, negligible overshoot, good disturbance rejection, and low controller effort for grid-tied converters with LCL filter. The developed solution uses a direct discrete-time pole-placement strategy from the classical control theory (using transfer functions), involving two extra filters, to enhance the performance of the RC. In this manner, the complexity of state-space methods from modern control theory is avoided. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.Ministerio de Ciencia e InnovaciónAgencia Estatal de Investigación | Ref. DPI2016-75832-RMinisterio de Economía y Competitividad | Ref. BES-2013-06314

Generalized multifrequency current controller for grid-connected converters with LCL filter

This paper presents a grid-side current controller for grid-tied inverters with LCL filter, including harmonic current elimination. The proposed controller only measures the grid current and voltage and it combines excellent dynamic characteristics with good robustness. Contrarily to previously proposed harmonic-current controllers, the presented solution offers a generalized method that gives a consistent (with minimal variation in the reference-tracking dynamics) and stable performance irrespectively of the number of current harmonics to be canceled and of the resonant frequency of the LCL filter (provided that it is lower than the Nyquist frequency). The response to reference commands is completely damped and fast. The response speed is set in accordance with the low-pass characteristic of the LCL filter so as to limit the control effort. Concerning the disturbance rejection, the controller offers an infinite impedance to any disturbances (such as grid voltage harmonics) at a set of arbitrarily specified frequencies. This allows the designer to eliminate all the undesired current harmonics with a simple design process. In addition, the performance of the presented controller is evaluated in terms of a fundamental tradeoff that exists between robustness to variations in the grid impedance and the number of frequency components rejected. Finally, simulation and experimental results that validate the proposal are presented.Ministerio de Ciencia e InnovaciónEuropean CommissionAgencia Estatal de Investigación | Ref. DPI2016-75832-RMinisterio de Educación, Cultura y Deporte | Ref. FPU14/0068

The use of crossbreeding with beef bulls in dairy herds: effects on calving difficulty and gestation length

This study was designed to analyse the evolution in the use of beef bull semen for dairy cattle insemination and, mainly, to assess calving difficulty, gestation length and proportion of stillbirths after breeding pure Holsteins or crossbreeding. Data were collected during 2004 to 2011 for 552 571 Holstein calvings (457 070 Holstein × Holstein, 43 384 Holstein × Limousine, 32 174 Holstein × Belgian Blue and 19 943 Holstein × Galician Blonde). The highest calving difficulty, compared with pure Holsteins was for crosses with Belgian Blue followed by Limousine and Galician Blonde. The Holstein × Limousine and Holstein × Galician Blonde crossbred calves had significantly longer gestation lengths than Holstein × Holstein and Holstein × Belgian Blue calves. Between the latter two, pure Holstein had the shortest gestation length. Calving difficulty and gestation length decreased as the age of the dam advanced. The most difficult calvings were observed in twin calvings, followed by the calvings of male calves and female calves. The gestations leading to the birth of male calves were longer than those leading to female calves and twin calves. Stillbirths were not related to the breed used for mating. Through examining these parameters, sire breed should be considered when selecting a beef breed for the insemination of milk-producing damsS

The use of physical restraints in long-term care in Spain: a multi-center cross-sectional study

Background Physical restraint is a procedure used frequently in long-term care. It is a controversial practice because its use is associated with numerous complications and also affects freedom and individual autonomy. The objective of this study was to examine the use of physical restraint of long-term care residents with the ability to move voluntarily. Methods We conducted a cross-sectional observational and correlational multi-center study. Nine centers agreed to participate. Of the 1,200 people present at the time of data collection, those without voluntary movement or in the facility for less than a month were excluded. Thus, the final sample was 920 residents. Data on the use of restraints was collected by direct observation. Information about the age, gender, length of stay, falls, mobility, cognition and functional status of residents was gathered by reviewing clinical records and interviewing nursing staff. A descriptive analysis of the data obtained was conducted. The generalized linear model was used, considering only the principal effects of each variable and using the logit link function. The model has been adjusted for clusters and for other possibly confounding factors. For all analyses, a confidence interval (CI) of 95% was estimated. Results The prevalence of residents with at least one physical restraint was 84.9% (95% CI: 81.7–88.1), with variability between centers of 70.3 to 96.6% (p-value Kruskal Wallis test <0.001). Full-enclosure side rails were most often used (84.5; 95% CI: 81.1–87.9), but other types of restraints were also used frequently. Multivariate analysis showed that the degree of functional impairment increased the probability of the use of restraint. A significant association was also found between restraint use and the impaired cognitive status of residents. Conclusions The prevalence was higher than in studies from other countries. The results emphasize the need to improve the training of nursing staff in the care of residents with impairments in functional and cognitive status. The use of alternative devices and nurse consultants need to be evaluated, and the introduction of specific laws considered.Sociedad Canaria de Geriatría y Gerontología and the Fundación Canaria de Investigación Sanitaria financed this study.S

Space-vector PWM with common-mode voltage elimination for multiphase drives

Switching common-mode voltage (CMV) generated by the pulse width modulation (PWM) of the inverter causes common-mode currents, which lead to motor bearing failures and electromagnetic interference problems in multiphase drives. Such switching CMV can be reduced by taking advantage of the switching states of multilevel multiphase inverters that produce zero CMV. Specific space-vector PWM (SVPWM) techniques with CMV elimination, which only use zero CMV states, have been proposed for three-level five-phase drives, and for open-end winding five-, six-, and seven-phase drives, but such methods cannot be extended to a higher number of levels or phases. This paper presents a general (for any number of levels and phases) SVPMW with CMV elimination. The proposed technique can be applied to most multilevel topologies, has low computational complexity and is suitable for low-cost hardware implementations. The new algorithm is implemented in a low-cost field-programmable gate array and it is successfully tested in the laboratory using a five-level five-phase motor drive.Ministerio de Ciencia e InnovaciónEuropean CommissionMinisterio de Economía y Competitividad | Ref. DPI2012-31283Ministerio de Economía y Competitividad | Ref. DPI2015-6541

Carrier-Based PWM equivalent to multilevel multiphase space vector PWM techniques

The space vector pulsewidth modulation (SVPWM) techniques enhance the performance of multilevel multiphase inverters. With multilevel (three-phase) inverters and with (two-level) multiphase inverters, it is widely accepted that the typical SVPWM strategies have an equivalent carrier-based pulsewidth modulation (CBPWM) counterpart, which produces identical results. However, the conclusions reached in the articles that show these cannot be applied, nor even extended, to SVPWM techniques with more than two levels and three phases. This article shows that the most widely accepted multilevel multiphase SVPWM techniques have a fully equivalent CBPWM counterpart, which consists of a phase disposition pulsewidth modulation with an appropriate zero-sequence injection scheme. Closed-form expressions to calculate the zero sequences are provided. The proposed modulation techniques are simulated and then implemented in a field- programmable gate array, showing that the equivalent CBPWM techniques produce identical results as the original SVPWM ones, but with a significant reduction of hardware requirements. The proposed methodology can be generalized to other multilevel multiphase SVPWM techniques.European CommissionEuropean Regional Development Fund | Ref. DPI2016-75832Ministerio de Ciencia e Innovación (España